Here are some points I pulled from all three articles and other online stuff. I hope it's correct.
Last time his mother heard from him was on August 15th. He phoned his mother from his great aunts house in Miami.
Checked out of his Miami Hotel (The Singapore Resort Hotel) on August 17th
He checked out of the hotel in time to make his flight which was scheduled for 11:15am
The address of the Singapore hotel was 9601 Collins Ave, Bal Harbour, FL 33154, USA which was a 25 minute drive to the Miami International Airport.
His rental car was returned to the rental car agency at (6:00pm) which was about 7 hours after he was supposed to board the 11:15am flight home. From the article, it looks like the rental car agency was located at the airport.
His airline ticket was not used.
May have had $1,200 dollars in gems (he bought them to put in a ring for his mother) and possessions.
His cards and accounts were never touched EXCEPT for three months after his disappearance. One of his credit cards was being used by a young couple in the New York Bronx district. Police missed catching the couple by 4 minutes.
A week before his disappearance there was a lot of rioting in Miami.
The Freedomtown riots took place on 11 August 1980 when hundreds of Cuban refugees began rioting at a detention center (nicknamed "Freedomtown") under the Interstate 95 in Downtown Miami in Florida, United States, burning tents and attacking immigration and nationalization guards with pipes, sticks, and rocks.
On August 16th, a day before Sandy went missing, There were 2 separate plane hijackings at the Miami Airport.
Weather on August 17th, 1980 was
Lows of 79.0 F
Highs of 89.1 F